Brake disks excellent in high speed and high load stabilities are adopted for cars and have come to be adopted also for autobicycles or motorcycles due to their excellent brake performances.
The brake disks of such brakes are required to be heat-treated to be of a hardness required in consideration of the brake feeling characteristics, wear-resistance, and prevention of squeaking sounds of the brakes. Further, together with such hardness control, it is required to set and maintain at a high precision the parallelism and flatness of the disk portion, i.e., the pad sliding surface in sliding contact with the brake pad.
Not only the brake disk, but also the clutch plate, is required to have precision in the parallelism and flatness of the surface in addition to the surface hardness. Sprocket wheels and other devices also require such good qualities.
Such products are made from a sheet-shaped member. Even if the precision of the parallelism and flatness of the surface is set in advance, due to heat-treatments such as quenching and annealing, the surface will be strained and deformed and it will be difficult to obtain and/or maintain the precision in parallelism and flatness. Also, if the material is punched or draw-shaped in advance, the precision of the dimensions will decrease.
The brake disk is of a type wherein an annular disk portion forming a pad sliding surface, and a hub fitting portion are integrally shaped, or a type wherein they are separately shaped. Both types are selectively used depending on the size of the vehicle.
In both types of such brake disks, the annular disk portion on the outer periphery forming the pad sliding surface is required to be wear-resistant, to have a proper hardness due to such brake feeling performance as a slip, and to have very high precision in parallelism and flatness of the sliding friction surface. Further, in case the brake disk is used for an autobicycle, it will be exposed to the elements, and rainwater or the like will enter the friction surface, and it will therefore be required to be anticorrosive.
In a prior method of making such brake disk, a stainless steel plate material is heated to a quenching temperature, is put between the upper and lower dies of a press, is held to be prevented from being thermally deformed, is here hot-draw-shaped or punched, is then quenched to be press-quenched, and is annealed. A strain may be produced by such two heat-treatments or, as such shaping and heat-treatment are made only by holding the plate material, the irregularities on the material surface will not be removed. Therefore, the material surface must be corrected by mechanical operations, such as grinding and cutting, to increase the precision of the parallelism and flatness of the annular disk portion and to obtain a predetermined brake disk.
Such prior method requires quenching and annealing steps in the producing process, has therefore many heat-treating steps, and is not adapted to mass production. As the material is quenched and annealed while being held only to prevent deformation, the disk portion or the disk portion and hub fitting portion in the integrally shaped disk will be reduced and strained in parallelism and flatness. Because only the surface is held, concavo-convexities will be produced by the deformation by the strain, quenching and heating. The concavo-convexities of the material itself are not removed, and the thickness dimensions will be incorrect or fluctuate. It will be difficult to obtain precision in the flatness of the surface, and the proper sliding friction surface with the brake pad will not be able to be attained. To obtain precision in parallelism and flatness, after the shaping, mechanical operations, such as grinding and cutting, are required. Therefore, the number of steps will increase. The surface hardness of the shaped product is so high that tool life will be short in the mechanical operation. More production equipment will be required due to the above.
When the plate thickness of the material does not match the plate thickness required for the product, i.e., in case the plate thickness of the material is larger than the required plate thickness or is particularly considerably larger, with the prior method a predetermined product will not be able to be substantially obtained. A material of a rough plate thickness cannot be used for a product required to have precision in the plate thickness dimension. If a material of a predetermined thickness dimension is made in advance instead of a material of such rough plate thickness, the cost of the material will undesirably become so high as to affect the cost of such product. If the predetermined thickness dimension is obtained by mechanically working the material instead of the above, it will not be adapted to mass-production and will increase the cost. Further, the plate thickness of the material is different depending on the kind of the brake disk, i.e., on the kind of vehicle to be fitted with the brake disk. The plate thickness dimension required for the final product is different depending on the respective products, such as clutch plates, sprockets, etc., and is difficult to obtain with a standardized plate material. Therefore, it is desirable to obtain such products while maintaining the precision of the plate thickness dimension by selecting a plate material thicker than the final plate thickness dimension irrespective of whether or not such thicker plate material matches the final plate thickness dimension.